1. Field of the Invention
The invention relates to the field of micromachined accelerometers and seismometers, and in particular to accelerometers and seismometers which can withstand high shock loads.
2. Description of the Prior Art
The measurement of the vibration of celestial bodies is one of the primary methods for determining their mechanical structure. At the planetary level, seismology has already successfully elucidated the internal structure of the Earth and moon. A long-running goal of planetary exploration has been similarly to determine the internal structure of Mars. However, an unambiguous seismic survey of the global structure of the planet requires a long-lived network distributed across the planet. The associated costs of deploying and running conventional seismometers precludes their use in any kind of Martian application.
Seismometers can also determine structure at a more local level and are used on Earth for surveying petroleum reservoirs. Proposals also exist for local seismic surveys of buried liquid water deposits on Mars. For small objects, determination of the vibration spectrum from a single sensor can be used to determine the mechanical structure of the object. Such an application is being considered to determine the relative thickness of the possible ice layer which has been observed on Europa, one of the major moons of Jupiter. A seismic investigation of a comet has been also planned under the name of the Rosetta Mission, which is projected to involve the first landing on the comet.
Common to all these applications is the need for a small, robust and low-powered seismometer with performance comparable to presently available terrestrial seismometers, namely sensitivity to signals below 1 ng/Hz. In order to meet this sensitivity, terrestrial seismometers have low-resonant-frequency suspensions, and this is resulted in bulky, massive instruments which are extremely delicate. Raising the resonant frequency of this seismometer allows a more compact instrument, but is achieved only at the expense of sensitivity.